The very reactive [B(CN)(3)](2-) dianion has a strongly nucleophilic boron atom and can be used for the synthesis of tricyanoborates,, which otherwise are difficult to:access.Herein'the reaction of this anion with HgCl2 is reported.,The main product is the anionic mercury complex- [Hg(B(CN)(3))(2)](2-). Hetetonutlear NMR spectroscopic experiments shows that the reaction proceeds via the intermediate [ClFIgB(CN)(3)](2-). Even though [Hg(B(CN)(3))(2)](2-) is the main product, it is difficult to obtain it in pure form, because it slwly decomposes in the presence of water and air to KNC)HgB(CN)(3)](-). All three anions were fully characterized- by hetereonudear NMR spectroscopy (B-11, C-13, and Hg-199). Single-crystal X-ray diffraction 'studies of the salts K[ClHgB(CN)(3)], [Ph4P](2)[Hg(B(CN)(3))(2)], K[(NC)HgB(CN)(3)], and [Ph(4)p][(NC)HgB(CN)(3)] revealed linear coordination environments, around mercury for all anions. The Hg-B bonds range from 2.219(5) angstrom in [Hg(B(CN)(3))(2)](2-) to 2.148(11)angstrom [ClHgB(CN)(3)](-), are in accord with the sum of the covalent radii of mercury,and boron, and can- be deScribed as covalent single bolds: A comparison with related, complexes indicates that :[B(cN)(3)](2-) dianion, is a stronger ligand than chloride, cyanide, or carbenes. [Hg(B(CN)(3))(2)](2-) hydrolyses in solution only in the presence of oxygen., It is suggested that,cis-[Hg(OH)(2),(B(CN)(3))(2)](2-) is fotmed as, a very unstable intermediate; which decomposes very fast to [(NC)HgB(CN)(3)](-) and other,productsThe anion cis-[Hg(OH)(2)(B(CN)(3))(2)](2-) would contain mercury in the unusual oxidation state +IV. Quantum-chemical calculations wete performed to support this assumption.